English Channel Towed Sledge Seabed Images. Phase 2: Analysis of Selected Tow Images
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ENGLISH CHANNEL TOWED SLEDGE SEABED IMAGES. PHASE 2: ANALYSIS OF SELECTED TOW IMAGES. Keith Hiscock Judith Oakley The Marine Biological Association of the UK wishes to acknowledge the financial support of the Joint Nature Conservation Committee This work will contribute to the MESH project (www.searchmesh.net) and received European Regional Development Funding through the INTERREG III B Community Initiative (www.nweurope.org). Report to the Joint Nature Conservation Committee October 2005 Reference: Hiscock, K., & Oakley, J., 2005. English Channel towed sledge seabed images. Phase 2: Analysis of selected tow images. Report to the Joint Nature Conservation Committee from the Marine Biological Association. Plymouth: Marine Biological Association. JNCC Contract F90-01-784. English Channel towed sledge seabed images. Phase 2: Analysis of selected tow images 2 English Channel towed sledge seabed images. Phase 2: Analysis of selected tow images English Channel towed sledge seabed images. Phase 1: scoping study and example analysis. CONTENTS EXECUTIVE SUMMARY.................................................................................................... 5 1. Introduction..................................................................................................................... 7 2. Methods.......................................................................................................................... 7 2.1 Images selected for analysis..................................................................................... 7 2.2 Methods of analysis .................................................................................................. 7 2.4 Comparison with other studies.................................................................................. 8 3. Results......................................................................................................................... 10 4. Discussion and comparison with late 19th and early 20th century descriptions ............. 30 4.1 Comparisons with late 19th and early 20th century descriptions .............................. 30 4.2 Comments on image analysis ................................................................................. 30 4.3 Accuracy of positioning and tabulation of positions................................................. 30 5. Thoughts on utility of the images for identifying change ............................................... 31 6. References ...................................................................................................................31 Appendix 1. Extracts from the Plymouth Marine Fauna................................................... 33 Appendix 2. Seabed infaunal communities sampled by Ford (1923) and matched to epifaunal towed sledge surveys........................................................................................ 35 3 English Channel towed sledge seabed images. Phase 2: Analysis of selected tow images 4 English Channel towed sledge seabed images. Phase 2: Analysis of selected tow images English Channel towed sledge seabed images. Phase 2: analysis of selected tow images. EXECUTIVE SUMMARY During the 1970s and 1980s, the late Dr Norman Holme undertook extensive towed sledge surveys in the English Channel and some in the Irish Sea. Only a minority of the resulting images were analysed and reported before his death in 1989 but logbooks, video and film material has been archived in the National Marine Biological Library (NMBL) in Plymouth. A study was therefore commissioned by the Joint Nature Conservation Committee and as a part of the Mapping European Seabed Habitats (MESH) project to identify the value of the material archived and the procedure and cost to undertake further work (Phase 1 of the study reported here: Oakley & Hiscock, 2005). Some image analysis was undertaken as a part of Phase 1. Phase 2 (this report) was to further analyse selected images. Having determined in Phase 1 that only the 35 mm photographic transparencies provided sufficient clarity to identify species and biotopes, the tows selected for analysis were ones where 35mm images had been taken. The tows selected for analysis of images were mainly in the vicinity of Plymouth and especially along the area between Rame Head and the region of the Eddystone. The 35 mm films were viewed under a binocular microscope and the taxa that could be recognised recorded in note form. Twenty-five images were selected for inclusion in the report. Almost all of the images were of level sediment seabed. Where rocks were included, it was usually unplanned and the sled was hauled before being caught or damaged. The main biotopes or biotope complexes identified were: SS.SMU.CSaMu. Circalittoral sandy mud. Extensively present between the shore and the Eddystone Reef complex and at depths of about 48 to 52 m. At one site offshore of Plymouth Sound, the turret shell Turritella communis was abundant. In some areas, this biotope had dense anemones, Mesacmaea mitchelli and (more rarely) Cerianthus lloydii. Queen scallops, Aequipecten opercularis and king scallops, Pecten maximus, were sometimes present in small numbers. Hard substratum species such as hydroids, dead mens fingers Alcyonium digitatum and the cup coral Caryophyllia smithii occurred in a few places, probably attached to shells or stones beneath the surface. South of the spoil ground off Hilsea Point at 57m depth, the sediment was muddier but is still assigned to this biotope complex. It is notable that three small sea pens, most likely Virgularia mirabilis, were seen here. SS.SMx.CMx. Circalittoral mixed sediment. Further offshore but at about the same depth as SS.SMU.CSaMu occurred, coarse gravel with some silt was present. The sediment was characterised must conspicuously by small queen scallops, Aequipecten opercularis. Peculiarly, there were ‘bundles’ of the branching bryozoan Cellaria sp. – a species normally found attached to rock. It could not be seen whether these bundles of Cellaria had been brought-together by terebellid worms but it is notable that Cellaria is recorded in historical surveys. As with many other sediments, there were occasional brittle stars, Ophiocomina nigra and Ophiura ophiura. Where sediments were muddy, the burrowing anemone Mesacmaea mitchelli was common. Where pebbles or cobbles occurred, there were attached species such as Alcyonium digitatum, Caryophyllia smithii and the fleshy bryozoan Alcyonidium diaphanum. Undescribed biotope. Although most likely a part of SS.SMx.CMx, the biotope visually dominated by a terebellid worm believed to be Thelepus cincinnatua, is worth special 5 English Channel towed sledge seabed images. Phase 2: Analysis of selected tow images attention as it may be an undescribed biotope. The biotope occurred about 22 nautical miles south of the latitude of the Eddystone and in depths in excess of 70 m. SS.SCS.CCS.Blan. Branchiostoma lanceolatum in circalittoral coarse sand with shell gravel at about 48m depth and less. This habitat was the ‘classic’ ‘Eddystone Shell Gravel’ which is sampled for Branchiostoma lanceolatum. However, no Branchiostoma lanceolatum could be seen. The gravel was almost entirely bare of epibiota. There were occasional rock outcrops or cobbles which had epibiota including encrusting calcareous algae, the sea fan Eunicella verrucosa, cup corals, Caryophyllia smithii, hydroids and a sea urchin Echinus esculentus. The variety of species visible on the surface is small and therefore identification to biotope not usually possible. Historical records from sampling surveys that used grabs and dredges at the end of the 19th century and early 20th century suggest similar species present then. Illustrations of some of the infaunal communities from work in the 1920’s is included in this report to provide a context to the epifaunal photographs. Reference Hiscock, K., & Oakley, J., 2005. English Channel towed sledge seabed images. Phase 2: Analysis of selected tow images. Report to the Joint Nature Conservation Committee from the Marine Biological Association. Plymouth: Marine Biological Association. JNCC Contract F90-01-784. 6 English Channel towed sledge seabed images. Phase 2: Analysis of selected tow images English Channel towed sledge seabed images. Phase 2: analysis of selected tow images. 1. INTRODUCTION Some image analysis was undertaken as a part of Phase 1. Phase 2 (this report) was to further analyse selected images. Many of the rolls of 35mm film show similar bottom type for the majority of the tow. Therefore precise matching of position and frames is not necessary to characterise the seabed along the tow. 2. METHODS 2.1 Images selected for analysis Having determined in Phase 1 that only the 35 mm photographic transparencies provided sufficient clarity to identify species and biotopes, the tows selected for analysis were ones where 35mm images had been taken. Tows were further selected as near to the Eddystone Reef complex, where there is current activity to map seabed habitats, offshore of Rame Head, the only area where mud was expected to be present and where there is current concern about dredge spoil disposal, and other locations to give a ‘spread’ to the samples. 2.2 Methods of analysis The films for each of the selected areas were rapidly inspected and one example tow from similar ones was set-aside for detailed inspection. Each frame of each film of the selected tows was inspected under a binocular microscope and notes made of substratum character and species present that could be seen